Before posting, consider that this is the definition of Atheism most commonly used in this sub, to avoid confusion and threads being snowballed by "That's not what Atheism means!"

There are many definitions of the word atheist, and no one definition is universally accepted by all. There is no single 'literal' definition of atheist or atheism, but various accepted terms. However, within non-religious groups, it is reasonable to select a definition that fits the majority of the individuals in the group.
For r/DebateAnAtheist, the majority of people identify as agnostic or 'weak' atheists, that is, they lack a belief in a god.

They make no claims about whether or not a god actually exists, and thus, this is a passive position philosophically.

The other commonly-used definition for atheist is a 'strong' atheist - one who believes that no gods exists, and makes an assertion about the nature of reality, i.e. that is it godless. However, there are fewer people here who hold this position, so if you are addressing this sort of atheist specifically, please say so in your title.